1. Field of the Invention
The present invention relates to an optical system of exposure apparatuses used in the manufacture of a semiconductor device and, more particularly, to a catadioptric optical system for lithography including a spherical mirror and lenses for transferring a pattern from an enlarged original mask onto a wafer using an ArF excimer laser as a light source, thereby realizing a very fine line width.
2. Description of the Prior Art
As the integration of the semiconductor devices with enlarged capacity has been improved, there is an increase in the demand for a highly fine pattern constituting a circuitry and a large chip size. In order to satisfy such a demand, there have been occurred necessities for higher performance of exposure apparatuses used in the semiconductor exposure process. The line width realized by exposure apparatuses fundamentally depends upon the resolution of optical system. The resolution of optical system in an exposure apparatus is proportional to the wavelength of a light source used and in an inverse proportion to the numerical aperture of optical system. In order to obtain a very fine pattern, therefore, it is necessary to use a light source having a short wavelength or to increase the numerical aperture of optical system. An increase in the numerical aperture of optical system is disadvantageous in regards to process and design of the optical system in that the depth of focus becomes small. For this reason, decreasing the wavelength of light source is preferable. However, where a short-wavelength ultraviolet light source is used, there is a limitation upon optical materials.
A conventional dioptric optical system for lithography is constituted by lenses only, and has to employ fused quartz exclusively as an optical material, which causes an astigmation. It is thus required to use a light source which operates in a short bandwidth enough to be acceptable with astigmation, in order to solve the above-mentioned problem. Other disadvantages involving such a conventional dioptric optical system are a rapid decline in the total transmittance of the optical system resulting from a loss of light since fused quartz exhibits a high absorptance, and a deterioration of the optical material by light source, thereby causing a defect in the fused quartz and reducing the life of optical system.